The invention relates to a process for reducing oxidic iron ores using solid highly volatile reductants in a tubular rotary kiln in which oxygen-containing gas or oxygen is introduced at locations distributed throughout the length of the kiln, e.g., by way of appropriately distributed and regulated nozzles, wherein at least part of the reductants are introduced from the discharge end of the kiln and distributed over a substantial region of the kiln charge bed and any remaining reductants are introduced at the feed end of the kiln with the oxidic iron ores and additives.
In the known processes of this type, in which iron ores in lump or pellet form are reduced in a tubular rotary kiln to form sponge iron, a mixture of iron ore, coal, crude dolomite or limestone serving as a desulphurizing agent, and excess carbon recovered from the process (recycle char) are fed into the rotary kiln mounted at an incline. The mixture of solids is heated by hot kiln gases to a temperature above 900.degree. C. to a maximum of 1240.degree. C. Such a process is disclosed, for example, in U.S. Pat. No. 3,890,138.
In the known processes the energy required for heating the solids, the heat requirements for the endothermic reaction, and the make-up for lost heat are generated partly by the post-combustion of carbon monoxide which is formed by means of oxygen in air introduced at various locations over the length of the kiln.
It is typical in all such processes that the temperature difference between the hot gases in the open kiln space and the kiln charge amounts to 100.degree.-150.degree. C. over a major part of the kiln length. It is moreover typical for all direct reduction processes conducted in rotary kilns to have very little carbon monoxide liberated by the charge in the lower kiln region, where the reduction has already been substantially completed. The small amount of carbon monoxide liberated is not sufficient to keep the temperature in that region of the kiln sufficiently high for a final metallization of more than 90% (metallization=metallic iron content/total iron content). In those processes, the additional thermal energy required in the discharge region of the rotary kiln is provided by the combustion of oil, gas or coal.
The rate at which the reaction proceeds and, accordingly, the amount of sponge iron produced per unit of kiln volume increases as the temperature of the solid mixture in the kiln is increased. Accordingly, for purposes of economical operation, the temperature of this material is set as high as possible. However, in those processes in which a major proportion of the small sized highly volatile coal is blown into the kiln from the discharge end, operating breakdowns frequently occur due to the adherence of molten or caking solid particles to the inner kiln periphery. This deposit buildup can result in a substantial interference with the material flow in the kiln. In various commercial plants they have resulted in such reductions of the free kiln cross section that production had to be stopped in order to strip off deposits from the kiln.